||Exercising in water has become widely accecpted as a popular training program in many athletic gymnasiums. However, surprisingly little information regarding the effects of water immersion, in a heate...d pool, on cardiovascular responses has been reported to date. Therefore, the cardiovascular responses during bathing in eight healthy men, aged 18-20(mean 19ys), werestudied using the M-mode and pulsed Doppler echocardiography. Bathing was conducted with subjects in the standing position(water depth : each subject's xiphoid level)in a heated poolat 3O℃ . The hear t rate, left atrial dimension(LAD), left ventricular end-diastolic dimension(LVDd) and left ventricular end-systolic dimension(LVDs) standing in air and during water immersion were compared. Furthermlre, to study the effect of water immersion on the left ventricular inflow, the subjects were evaluated with pulsed Doppler echocardiography, which was performed with the sample volume placed at the mitral leaflet tips in the apical 4-chamber projection. The LV inflow measurements comprised peak early filling(E) and peak atrial filling velocities(A) .The measurements were recorded at the baseline(standing in air)and during water immersion in a standing position. Water immersion produced a significant decrease in the heart rate(HR) from 71.4±7.8 to 57.4±4.3 beats/ min(mean±standard deviation) , p=0.0005, but the blood pressure remained unchanged. Water immersion produced a significant increase in the LAD(2.3±0.3 to 3.1±0.3cm, p=0.0001) ,LVDd (4.3±0.3 to 5.2±0.2cm, p=0.0001) and LVDS (3.2±0.3 to 3.7±0.2 cm, p =0.0003). The peak early filling velocity(E) did not change significantly(0.64±0.08to 0.74±0.15m/sec) . The peak atrial velocity(A) decreased from 0.34±0.08 to 0.26±0.06m/sec(p<0.05), while the A/E ratio decreased 0. 54±0.10 to 0.37±0.12m/sec (P < 0.005). These results sugggest that(1)the increase in the venous return when immersing a standing subject in water produced significant inceases in the LAD, LVDd, LVDs, and stroke volume, (2)the increase in the chamber sizes may cause a decrease in the heart rate, (3) the increase in the diastolic interval might reduce the A and A/E ratio. It is concluded that water immersion at30℃ may thus result in a volume overload to the subjects, and not a pressure overload.